1. Field of the Invention
The present invention relates to a battery voltage detection device suitable for use in a photographic device, and, more particularly, the present invention relates to a battery voltage detection device for a photographic device which uses a plurality of battery voltage measurements taken during pulse width modulation control to determine battery voltage.
2. Description of the Related Art
Cameras have in recent years become multifunctional devices having various sophisticated electronic components and systems which are supplied power by a battery built into the camera. However, the battery gradually becomes exhausted and the battery voltage falls as a result of operating various camera systems, such as autofocus ("AF"), automatic exposure ("AE"), strobe light generation, and the display of photographic information in the viewfinder. More specifically, the camera battery is used as a common power supply for an AF drive motor, shutter drive magnets, a mirror drive motor, a built-in strobe charging circuit, a circuit which drives an LED used for backlighting a liquid crystal display in the viewfinder, and the like. Further, the camera battery is used as a power supply for integrated circuits ("ICs") of a microcomputer, sensors, motor drivers, and the like, which control the above-described motors, circuits, etc.
A DC-DC converter converts the camera battery voltage to 5 V or 12 V, as appropriate, to supply power to the ICs. When the camera battery is exhausted and the battery voltage falls, resulting in a fall of the output voltage of the DC-DC converter, the ICs do not operate normally, and anomalies arise in the control of the autofocus system, the automatic exposure system, and the like. To prevent such anomalies caused by a fall in the battery voltage, battery check devices to check the state of exhaustion of the battery have been developed.
In accordance with a known battery check device, it is determined whether photography is possible by detecting the battery voltage at constant time intervals during a predetermined time period (for example 8 seconds) starting from when a release button is half-depressed. The above-described detection of the battery voltage is not performed during an AF operation or during charging of the built-in strobe, when there is a temporary fall of the battery voltage. However, a problem occurs in the known battery check device because the camera battery is used to supply power to various camera systems and devices, and the battery voltage is subject to the effects of changes in the operation of circuits. Moreover, error components are usually included in the battery detection result, and as a result the battery voltage can not be accurately detected.
For example, the camera battery voltage changes with the operation of a light emitting diode ("LED") used for backlighting a liquid crystal display in the viewfinder. Specifically, the ON/OFF ratio of the LED used for backlighting the liquid crystal display in the viewfinder is set by pulse width modulation ("PWM") control. Further, when PWM control performs the switching ON and OFF of the LED for liquid crystal backlighting, the current flowing to the LED changes greatly when the LED is switched ON and when the LED is switched OFF.
FIG. 4 is a graph showing the change in measured battery voltage with respect to time as affected by PWM control. In FIG. 4, T.sub.0 denotes a period during which the voltage of the battery is caused to change when PWM control is performed. For example, a large difference results between the measured battery voltage value m.sub.11 measured at the instant a.sub.1, and the measured battery voltage value m.sub.21 measured at the instant a.sub.2. Furthermore, when a filter circuit or the like is used to eliminate the variation in measured battery voltage caused when the PWM control sets the ON/OFF ratio of the LED used for backlighting a liquid crystal display, the cost of the camera is increased.